The present invention relates to an improved method and apparatus for simultaneously terminating multiple electrical leads to multiple electrical terminals. Although the description set forth below mentions solder as the preferred fusible joining material, it is to be understood that any appropriate fusible material may be employed, such as doped plastic material.
It is known in the prior art to terminate the individual conductors in a ribbon cable (i.e., a cable having parallel conductors in a side-by-side orientation) by means of insulation displacement or piercing techniques such as described in U.S. Pat. No. 3,820,055. Such techniques have limitations in that they are relatively unreliable for stranded wires and for wires having a high current carrying capacity. Another known termination technique involves stripping the insulation from the individual conductors and crimping the conductors to respective terminals. However, this technique is quite time consuming and not suitable for conductors that are very closely spaced.
Soldering individual cable leads to respective terminals can be achieved by a variety of prior art methods and apparatus. The least desirable of these is manually soldering each terminal and lead because the resulting repetitive soldering operations are time consuming and costly. In addition, if a large number of leads from a common cable are to be soldered, great care must be taken to avoid inadvertent application of heat from the soldering tool to previously soldered components, resulting in the weakening or destruction of the solder connection.
Prior art soldering procedures for forming multiple solder joints simultaneously employ a soldering tool to deliver the necessary thermal energy over a large continuous area spanning all of the connection sites. Upon energization, the soldering tool heats up until it overshoots a control temperature (i.e., the temperature at which the solder material melts) before settling down to that temperature. The control temperature is typically chosen somewhat above the ideal soldering temperature in order to compensate for less than ideal thermal energy transfer. This approach to thermal energy delivery has a number of disadvantages. For example, the thermal energy applied to spaces between the connection sites is wasted. Another disadvantage is the likelihood of damage to components resulting from overheating. More specifically, the thermal overshoot inherent in the heating tool can damage components disposed between the connection sites within the area heated by the tool. In some cases the overshoot may cause damage to the components to be joined at the connection site. It is tempting to suggest that the operator of the soldering tool might avoid the thermal overshoot by either removing the tool before the overshoot occurs or delaying application of the tool until after the overshoot occurs. This is impractical for a number of reasons. First, there is no evident indication as to when the thermal overshoot occurs. Second, although the tool warm-up time is quite long, the time interval during which the tool temperature is sufficient to melt solder, but prior to overshoot, is too short to reliably complete the soldering operation. On the other hand, leaving the soldering tool energized at its steady state temperature becomes expensive and wasteful of energy. Where the soldering tool is also employed to apply pressure to the connection site, the power must be turned off after the solder melts and pressure is applied until the solder solidifies.
It is desirable, therefore, to provide a method and apparatus employing a wire termination technique that is more reliable than insulation displacement and less time consuming than individually crimp-terminating conductors to terminals. Such method and apparatus should permit simultaneous soldering of multiple terminals to multiple leads at respective connection sites without applying thermal energy to spaces between those sites. In addition, it is desirable that the thermal energy required to melt the solder be available virtually instantaneously after energization of the heater, and that the heater be arranged to provide no more thermal energy than is required to melt the solder employed for the various connection sites.
It is known in the prior art to provide a plurality of electrical terminals which are stamped and formed in a stamping press from a continuous strip of metal, with a portion of the metal strip remaining integral with each of the terminals to serve as a carrier strip along which the terminals are spaced. The integral carrier strip allows for conveyance of the terminals in sequential relationship through an insertion machine, typically having a severing station where terminals are removed from the strip, and an insertion station where the removed terminals are inserted into a printed circuit board, a connector housing, or other work piece in which the terminals are either mounted or contained. Once they are so inserted, the terminals are soldered to individual wires from cables, and the like. It is also known, as described in U.S. Pat. No. 4,021,095 (Kinkaid et al), to provide a plurality of carrier strips adapted to be stacked together immediately prior to being simultaneously conveyed to an insertion machine. The stacked carrier strips are mutually offset such that the terminals thereof are arranged with closer spacing than are the terminals along any one of the stacked carrier strips. The present invention makes advantageous use of this carrier strip concept of interdigitated terminals. The present invention also makes use of a relatively new automatic self-regulating heater technology disclosed in U.S. Pat. Nos. 4,256,945 (Carter et al), 4,623,401 (Derbyshire et al), 4,659,912 (Derbyshire), 4,695,713 (Krumme), 4,701,587 (Carter et al), 4,717,814 (Krumme) and 4,745,264 (Carter). The disclosures in these patents are expressly incorporated herein by reference. A heater constructed in accordance with this technology, hereinafter referred to as a self-regulating heater, employs a substrate of copper, copper alloy or other material of low electrical resistivity, negligible magnetic permeability and high thermal conductivity. A thin layer of thermally-conductive magnetic material is deposited on all or part of one surface of the substrate, the magnetic material typically being an iron, nickel or nickel-iron alloy, or the like, having a much higher electrical resistance and magnetic permeability than the substrate material. A constant amplitude, high frequency alternating current is passed through the heater and, as a result of the skin effect phenomena, is initially concentrated in the thin alloy layer. If the temperature of that layer reaches the Curie temperature of the alloy, the magnetic permeability of the layer decreases dramatically, thereby significantly increasing the skin depth so that the current density profile expands into the non-magnetic substrate of low resistivity. The overall result is a lower resistance and lesser heat dissipation. If thermal sinks or loads are placed in contact with the heater at different locations along the heater length, thermal energy is transferred to the loads at these locations with the result that the temperature does not rise to the alloy Curie temperature as quickly at those locations as it does in the non-load locations. The constant amplitude current remains concentrated in the higher resistance alloy layer at the load locations which dissipate considerably more resistive heating energy than is dissipated in the non-load locations where the current is distributed in the low resistance substrate.